PHYSIOLOGY OF THE INVERTEBRATA. 63 



There are four principal muscles (see Figs. 14 and 1 5) which 

 work this complex stomach. The two anterior gastric muscles 

 attached to the cardiac ossicle, ascend obliquely forwards and 

 are fixed to the inner surface of the carapace. The two 

 posterior gastric muscles attached to the pyloric ossicle are 

 also fixed to the carapace. The food torn to pieces by the 

 mandibles is crushed into a fine state of division in the. 

 cardiac portion of the stomach. The thick walls of the 

 pyloric portion are covered internally with long hairs. These 

 project into the interior forming a kind of strainer, which only 

 allows the nutritive juices and finely divided particles to pass 

 into the intestine. 



At the sides of the cardiac portion of the stomach, embedded 

 in its tissues, are usually to be found in the summer two cal- 

 careous masses or plates, known as gastroliths. At the period 

 when the crayfish moults the gastroliths are also cast. They 

 weigh from two to three grains ; what their function may be 

 is still unknown. Possibly, they may be simply deposits due 

 to an excess of calcareous matter in the system.* 



We have now come to the end of the great class, Arthro- 

 PODA. The majority of its members, excepting degenerate 

 types, have well-defined digestive apparatuses. Often, as in 

 the Criidacca and T/iscda, the intestinal epithelium is furnished 

 with a hard layer of chitin (Cj^H,gNjO,J, sometimes raised 

 into projections destined to crush and macerate the food. The 

 mouth is either suctorial, masticatory, or biting ; and in the 

 Crastacm certain anterior parts of the intestinal canal become 

 buccal pieces. In some of the lower Arthropods there are 

 present both salivary glands and so-called livers ; in others, 

 either one or the other organ is absent — cjj., both salivary 

 glands and "livers" are present in the Orthoijtcra, Coleoptcra, 

 and Artlirogastra. In the Lcpidoidcra^ Ardisca, and Bilojwd a 

 only salivary glands are present ; and in the Xijiliosura and 

 Ostracoda there are onlv the so-called livers or digestive 



* See also Irvine and Woodhead in Proc. Eoij. Soc. Edin., \q\. 16, 

 P- 330. 



